1. Field of the Invention
Embodiments of the invention generally relate to thermal processing of substrates, and, more specifically, to apparatus and methods for automatically focusing energy onto and measuring emissivity of a substrate during thermal processing.
2. Description of the Related Art
During electronic device fabrication, substrates may be heated to high temperatures so that various chemical and/or physical reactions can take place. Thermal processes are usually used to heat the substrates. A typical thermal process, such as annealing, requires providing a relatively large amount of thermal energy to the substrate in a short amount of time, and thereafter rapidly cooling the substrate to terminate the thermal process. Examples of thermal processes currently in use include Rapid Thermal Processing (RTP) and impulse (spike) annealing. While such processes are widely used, current technology tends to ramp the temperature of the substrate too slowly and expose the substrate to elevated temperatures for too long. These problems become more severe with increasing substrate sizes, increasing switching speeds, and/or decreasing feature sizes.
In general, these thermal processes heat substrates under controlled conditions according to a predetermined thermal recipe. These thermal recipes typically consist of a temperature that the semiconductor substrate must be heated to, the rate of change of temperature (i.e., the temperature ramp-up and ramp-down rates), and the time that the thermal processing system remains at a particular temperature (sometimes referred to as “dwell time”). For example, thermal recipes may require the substrate to be heated from room temperature to temperatures of 1200° C. or more, for processing times at each temperature ranging up to 60 seconds, or more.
Moreover, to meet certain objectives, such as minimal inter-diffusion of materials between different regions of a substrate, the amount of time that each substrate is subjected to high temperatures must be restricted. To accomplish this, the temperature ramp rates, both up and down, are preferably high. In other words, it is desirable to be able to adjust the temperature of the substrate from a low to a high temperature, or vice versa, in as short a time as possible.
The requirement for high temperature ramp rates led to the development of Rapid Thermal Processing (RTP), where typical temperature ramp-up rates range from 200 to 400° C./s, as compared to 5-15° C./minute for conventional furnaces. Typical ramp-down rates are in the range of 80-150° C./s. A drawback of RTP is that it heats the entire substrate even though the integrated circuit (IC) devices reside only in the top few microns of the silicon substrate, which limits how fast one can heat up and cool down the substrate. Moreover, once the entire substrate is at an elevated temperature, heat can only dissipate into the surrounding space or structures. As a result, today's state of the art RTP systems struggle to achieve a 400° C./s ramp-up rate and a 150° C./s ramp-down rate.
FIG. 1 is a graph 100 of thermal profiles of different prior art thermal processes. As can be seen, the thermal profile 102 of a typical RTP system has a 250° C./s ramp-up rate and a 90° C./s ramp-down rate.
A drawback of RTP is that it heats the entire substrate even though the IC devices reside only in the top few microns of the substrate. The heating of the entire substrate limits how fast one can heat up and cool down the substrate. Moreover, once the entire substrate is at an elevated temperature, heat can only dissipate into the surrounding space of structures. As a result, today's state of the art RTP systems struggle to achieve 400° C./s ramp-up rates and 90° C./s ramp-down rates.
FIG. 1 also shows a thermal profile 104 of a laser annealing process. Laser annealing is used during the fabrication of thin film transistor (TFT) panels. Such systems use a laser spot to melt and recrystallize polysilicon. The entire TFT panel is exposed by scanning the laser spot across successive exposure fields on the panel. For substrate applications, a laser pulse is used to illuminate an exposure field for a duration of approximately 20-40 ns, where the exposure field is obtained by rastering across and down the substrate.
One laser annealing technique is known as dynamic surface annealing (DSA). In general, this technique delivers a constant energy flux to a small region on the surface of the substrate while the substrate is translated, or scanned, relative to the energy delivered to the small region. Due to the stringent uniformity requirements and the complexity of minimizing the overlap of scanned regions across the substrate surface, these types of processes may not be effective for thermal processing contact level devices formed on the surface of the substrate.
Pulsed laser anneal techniques, generally project pulsed electromagnetic energy at one small region on a substrate, and then move the substrate relative to the energy source and expose other small regions to pulsed electromagnetic energy. The pulsed laser anneal technique minimizes overlap between processing regions on the substrate, thereby improving thermal annealing uniformity. The energy sources used in the pulsed laser anneal techniques must be able to deliver a relatively large amount of energy at a relatively short time period.
Laser annealing techniques require the laser energy to be focused onto the substrate for optimal heating. Focusing the laser typically occurs at the initial setup, not during the thermal processing of each substrate. As a result, various factors may cause the laser to be slightly out of focus, which can cause non-uniform annealing of the substrate. For example, each substrate may be slightly tilted, causing different portions of the substrate to have different locations with respect to the focus plane of the laser. Individual substrates may vary in thickness by ±50 μm. The location of the focus plane may vary as a result of heating in the optical components.
Variations in emissivity of the substrate may affect the uniformity of the annealing of the substrate. An absorption layer is deposited on a substrate before thermal processing so that the substrate will absorb heat from the laser. The absorption layer may vary in composition and thickness, which can cause corresponding variations in emissivity. As a result, the amount of heat absorbed by the substrate from the laser will vary, resulting in non-uniform annealing of the substrate. If a pyrometer is used to measure temperature, variations in emissivity at a wavelength monitored by the pyrometer will affect temperature readings, potentially causing errors in temperature control.
In view of the above, laser annealing processes may be improved by correcting for the effects of variations in substrate and laser properties.